Axial flow fun

ABSTRACT

The invention is directed to dual purposes of increasing air volume and reducing noises of an inline axial flow fan. In the inline axial flow fan including a first axial flow fan unit  100 - 1 , a first honeycomb  200 - 2 , a second axial flow fan unit  100 - 2  and a second honeycomb  200 - 2  which are arranged in the order starting from an upstream side in an air flow direction, the first honeycomb includes a stator vane configured to be warped in a “U” shape against a rotation direction of the first axial flow fan unit, while the second honeycomb includes a stator vane configured to direct a trailing edge thereof in parallel to the air flow direction.

CLAIM OF PRIORITY

The present application claims priority from Japanese patent applicationserial no. 2010-163007 filed on Jul. 20, 2010, the content of which ishereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an axial flow fan including axial flowfan units serially arranged in a direction of rotary shafts thereof.

2. Description of Related Art

Home electric appliances and OA/IT apparatuses are equipped with acooling fan for cooling heat-generating electronic components. Morerecently, the market has been meeting demands for the downsizing andsophistication of these home electric appliances and OA/IT apparatuses.Along with the downsizing and sophistication efforts, the appliances andapparatuses tend to have an internal structure more densely mounted withthe electronic components. This results in the increase in the amount ofheat generation.

A compact, high volume axial flow fan is generally employed as a coolingfan to deal with the increased amount of heat generated by theelectronic components.

However, in a case where the compact axial flow fan is employed forcooling the electronic components, the fan must be rotated at highspeeds to provide a required volume of cooling air. Unfortunately, thisentails a problem of noise increase although the air volume is increasedby rotating the fan at high speeds.

On the other hand, a structure having the axial flow fan units seriallyarranged in the direction of rotary shafts thereof is adopted to dealwith the increase in pressure loss as a consequence of the high-densitymounting of electronic components.

As particularly exemplified by server apparatuses at data centers,machines and equipment designed on the assumption of long hours ofcontinuous operation adopt a structure having a plurality of axial flowfan units operatively arranged in series from the standpoint of ensuringredundancy for preventing the total breakdown of a cooling functionassociated with the failure of the cooling fan.

In the structure wherein the axial flow fan units are serially arrangedand operated, therefore, emphasis is placed on a technique for reducingnoises during the operation of outputting the increased volume ofcooling air.

U.S. Pat. No. 4,167,861 discloses a structure wherein two axial flow fanunits are serially arranged in the direction of rotary shafts thereof.Interposed between the upstream axial flow fan unit and the downstreamaxial flow fan unit is a device (hereinafter, referred to as“honeycomb”) including a frame and vanes. The frame is called a statorand includes an inside surface and an outside surface. The vanes extendradially from the center of the frame. This honeycomb removes a swirlingflow produced in an airflow by the axial flow fan unit, thus suppressingthe noise generation.

However, the honeycomb of the U.S. Pat. No. 4,167,861 does not work onthe air flow discharged from the downstream axial flow fan unit,although working on the air flow discharged from the axial flow fan unitdisposed upstream thereof.

Therefore, the swirling flow in the air flow discharged from thedownstream axial flow fan unit cannot be removed although theabove-described honeycomb acts to remove the swirling flow from the airflow discharged from the upstream axial flow fan unit. In view of thewhole body of the inline axial flow fan, therefore, the U.S. Pat. No.4,167,861 is not necessarily considered to provide an effective solutionto the above-described problem of noise generation.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an axial flow fan adapted toincrease the air volume and to reduce the noise of inline axial flow fanunits thereof.

The above object is accomplished in an axial flow fan comprising: afirst axial flow fan unit disposed on an upstream side with respect toan air flow; a first honeycomb disposed downstream of the first axialflow fan unit; a second axial flow fan unit disposed downstream of thesecond honeycomb; and a second honeycomb disposed downstream of thesecond axial flow fan unit, wherein a stator vane constituting the firsthoneycomb is configured to be warped against a rotation direction of thefirst axial flow fan unit while a stator vane constituting the secondhoneycomb is configured to direct a trailing edge thereof in parallel toa direction of the air flow.

The above object is further accomplished in the axial flow fan whereinthe stator vane constituting the first honeycomb is warped in a “U”shape.

The above object is further accomplished in the axial flow fan whereinthe stator vane constituting the first honeycomb is divided into twoparts.

The above object is further accomplished in an axial flow fancomprising: a first axial flow fan unit disposed on an upstream sidewith respect to an air flow; a first honeycomb disposed downstream ofthe first axial flow fan unit; a second axial flow fan unit disposeddownstream of the first honeycomb; and a second honeycomb disposeddownstream of the second axial flow fan unit, the second axial flow fanunit rotating in a different way from the first axial flow fan unit,wherein a stator vane constituting the first honeycomb is configured todirect a ventral side thereof against a rotation direction of the firstaxial flow fan unit, while a stator vane constituting the secondhoneycomb is configured to direct a trailing edge thereof in parallel toa direction of the air flow.

The above object is accomplished in the axial flow fan comprising aninline axial flow fan wherein the first and second axial flow fan unitsand the first and second honeycombs are used as a device for coolingserver apparatuses.

The invention can provide the axial flow fan adapted to increase the airvolume and to reduce the noise of the inline axial flow fan unitsthereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a structure wherein axial flow fanunits and honeycombs are alternately arranged in series;

FIG. 2 is a side view showing the axial flow fan unit;

FIG. 3 is a perspective view showing the axial flow fan unit;

FIG. 4 is a side view showing the honeycomb unit;

FIG. 5 is a perspective view showing the honeycomb unit;

FIG. 6 represents a cylindrical plane containing an inline axial flowfan according to a first embodiment of the invention;

FIG. 7 is chart showing a relation between air inflow velocity and airexit velocity for a rotor blade of the axial flow fan;

FIG. 8 is a graph showing performance curve and resistance curve of theaxial flow fan;

FIG. 9 is a diagram showing air flow separation caused by negativepreswirl;

FIG. 10 represents a cylindrical plane containing an axial flow fanaccording to a second embodiment of the invention;

FIG. 11 is a diagram showing a structure of an axial flow fan includingaxial flow fan units and honeycombs according to a third embodiment ofthe invention;

FIG. 12 represents a cylindrical plane containing the inline axial flowfan according to the third embodiment of the invention;

FIG. 13 is a diagram showing a structure of an axial flow fan includingaxial flow fan units and honeycombs according to a fourth embodiment ofthe invention;

FIG. 14 represents a cylindrical plane containing the inline axial flowfan according to the fourth embodiment of the invention; and

FIG. 15 is a schematic diagram showing a structure of a blade serveraccording to a fifth embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the invention will be described as below withreference to the accompanying drawings. Referring to FIG. 2 to FIG. 5, abrief description is made on an axial flow fan unit and a honeycombarranged in series.

FIG. 1 is a schematic diagram showing a structure wherein the axial flowfan units and the honeycombs are alternately arranged in series.

FIG. 2 is a side view showing the axial flow fan unit.

FIG. 3 is a perspective view showing the axial flow fan unit.

FIG. 4 is a side view showing the honeycomb unit.

FIG. 5 is a perspective view showing the honeycomb unit.

Referring to FIG. 1, a first axial flow fan unit 1, a first honeycomb 2,a second axial flow fan unit 3 and a second honeycomb 4 are arranged inseries in the order starting from an upstream side in an air flowdirection indicated by the arrows. Namely, the first axial flow fan unitis disposed on the upstream side while the first honeycomb 2 is disposeddownstream of the first axial flow fan unit 1. The second axial flow fanunit 3 is disposed downstream of this first honeycomb 2. The secondhoneycomb 4 is disposed downstream of this second axial flow fan unit 3.

Referring to FIG. 2 and FIG. 3, the first and second axial flow fanunits 1, 3 are centrally formed with a boss 101, respectively. Aplurality of rotor blades 102 are provided on an outer periphery of theboss 101. A motor 103 is coupled to the boss 101, which is brought intorotation by the motor 101 so as to rotate the rotor blades 102. Supportstruts 105 support the motor 103 on a casing 104.

Referring to FIG. 4 and FIG. 5, the first and second honeycombs 2, 4each include an inside frame 201 and an outside frame 202. The insideframe 201 and the outside frame 202 are interconnected by a plurality ofstator vanes 203 extending radially from the inside frame 201.

According to the above-described patent literature 1, the honeycomb 2 isinterposed between the first axial flow fan unit 1 and the second axialflow fan unit 3 but the second honeycomb 4 on the downstream side is notprovided. In the structure of the patent literature 1, therefore, aswirling flow can be removed from an air flow discharged from the firstaxial flow fan unit 1 by the effect of the honeycomb 2 but the swirlingflow cannot be removed from the air flow discharged from the secondaxial flow fan unit 3.

In this connection, the present inventors have achieved the followingembodiments by installing the second honeycomb 4 downstream of thesecond axial flow fan unit 3 and making various studies on theconfiguration of the stator vanes of the second honeycomb 4.

First Embodiment

FIG. 6 represents a cylindrical plane containing an inline axial flowfan according to a first embodiment of the invention.

Namely, FIG. 6 represents the cylindrical plane containing fragmentaryviews of cross sections of the rotor blades 102 of the first and secondaxial flow fan units 1, 3 and cross sections of the stator vanes 203 ofthe first and second honeycombs 2, 4 shown in FIG. 1.

In FIG. 6, as seen from an upstream side in an air flow directionindicated by the arrows, a rotary rotor blade 102 a of the first rotaryaxial flow fan unit 1 (hereinafter, referred to as “first rotor blade102 a”) is disposed on the upstream side. A stationary stator vane 203 aof the first honeycomb 2 (hereinafter, referred to as “first stator vane203 a”) is disposed downstream of this rotor blade 102 a. A rotary rotorblade 102 b of the second axial flow fan unit 3 (hereinafter, referredto as “second rotor blade 102 b”) is disposed downstream of this statorvane 203 a. A stationary stator vane 203 b of the second honeycomb 4(hereinafter, referred to as “second stator vane 203 b”) is disposeddownstream of the rotating rotor blade 102 b.

The first rotor blade 102 a and the second rotor blade 102 b rotate inthe same direction and have rotary shafts in aligned relation. The firststator vane 203 a is warped in a “U” shape against a rotation directionof the rotor blade 102 a and the rotor blade 102 b. The second statorvane 203 b is configured to direct a trailing edge thereof in parallelto the air flow direction.

These honeycombs 2, 4 allow the air flow to enter the rotor blade 102 aat a relative velocity 302 a for a rotational field and at an absolutevelocity 303 a for a static field. In the rotational field commonlyrepresented by the three-dimensional cylindrical coordinate system, therelative velocity is given as a sum of a circumferential velocity andthe absolute velocity.

Passing through the rotor blade 102 a, the air flow exits at a relativevelocity 302 b for the rotational field and at an absolute velocity 303b for the static field.

FIG. 7 is a chart showing a relation between air inflow velocity and airexit velocity for the rotor blade of a common axial flow fan.

Referring to FIG. 7, the air flow enters the rotor blade at a relativeinflow velocity 302(a) and a relative inflow angle 305(a) for therotational field, and at an absolute inflow velocity 303(a) and anabsolute inflow angle 304(a) for the static field. After passing throughthe rotor blade, the air flow exits at a relative exit velocity 302(b)and a relative exit angle 305(b) for the rotational field and at anabsolute exit velocity 303(b) and an absolute exit angle 304(b) for thestatic field. The air flow is varied in velocity as follows due to theeffect of the rotor blade. In the rotational field, a velocity variationis given by a difference 306(b) between the relative inflow velocity305(a) and the relative exit velocity 305(b). In the static field, avelocity variation is given by a difference 306(a) between the absoluteexit velocity 303(b) and the absolute inflow velocity 303(a).

P _(th) =ρu(v _(θout) −v _(θin))=ρu(w _(θin) −w _(θout))  Equation 1

The equation 1 represents the theoretical total pressure rise of the airflow provided by the effect of the rotor blade. In the equation,“P_(th)” denotes a theoretical total pressure rise; “ρ” denotes an airdensity; “u” denotes a circumferential velocity; “w_(θin)” denotes aswirl component of the relative inflow velocity; “w_(θout)” denotes aswirl component of the relative exit velocity; “v_(θin)” denotes a swirlcomponent of the absolute inflow velocity; and “v_(θout)” denotes aswirl component of the absolute exit velocity. The equation 1 means thatthe theoretical total pressure rise of the air flow is proportional tothe velocity variation of the air flow caused by the effect of the rotorblade.

In the above-described first rotor blade 102 a, a theoretical totalpressure rise corresponding to an inflow velocity and an exit velocityof the air flow through the first rotor blade 102 a of FIG. 6 can becalculated from the equation 1.

The air flow exiting from the first rotor blade 102 a enters the firststator vane 203 a at the absolute velocity 303 b for the static fieldand exits from the stator vane at an absolute velocity 303 c asdecelerated by the effect of the first stator vane 203 a. As aconsequence of the configuration of the first stator vane 203 a warpedin the “U” shape against the rotation direction of the second rotorblade 102 b, the absolute velocity 303 c contains a swirl component,called a negative preswirl, in the opposite direction to the rotationdirection of the second rotor blade 102 b.

$\begin{matrix}{{\Delta \; P_{s}} = {\rho \frac{{v^{2}{in}} - {v^{2}{out}}}{2}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

The equation 2 represents the theoretical static pressure rise in theair flow provided by a common effect of the stator vane. In theequation, “ΔP_(s)” denotes a theoretical static pressure rise; “ρ”denotes an air density; “v_(in)” denotes an absolute inflow velocity;and “v_(out)” denotes an absolute exit velocity. The equation 2indicates that the absolute velocity of the air flow is decreased by theeffect of the stator vane whereby the static pressure in the air flow isincreased.

In the first stator vane 203 a, a theoretical static pressure risecorresponding to an inflow velocity and an exit velocity of the air flowthrough the first stator vane 203 a of FIG. 6 can be calculated from theequation 2.

The air flow exiting from the first stator vane 203 a enters the secondrotor blade 102 b at a relative velocity 302 c for the rotational fieldand at the absolute velocity 303 c for the static field. The air flowpasses through the second rotor blade 102 b and exits at a relativevelocity 302 d for the rotational field and at an absolute velocity 303d for the static field. At this time, the swirl component of theabsolute inflow velocity in the equation 1 representing the theoreticaltotal pressure rise of the air flow has the negative sign. Therefore,the theoretical total pressure rise is increased in value as comparedwith a case where the swirl component of the absolute inflow velocityhas the positive sign. This effect permits the reduction of thecircumferential velocity when as much theoretical total pressure rise asthat of a case where the swirl component of the absolute inflow velocityhas the positive sign is imparted to the air flow.

L′ _(A) =L _(A)+60 log₁₀(N′/N)  Equation 3

The equation 3 represents the variation of noise level associated withthe variation of motor revolving speed. In the equation, “N” denotes apre-variation revolving speed; “N′” denotes a post-variation revolvingspeed; “L_(A)” denotes a pre-variation noise level; and “L′_(A)” denotesa post-variation noise level.

If the motor revolving speed is reduced by reducing the circumferentialvelocity of the second rotor blade 102 b, the noise level is lowered asindicated by the equation 3.

The air flow exiting from the second rotor blade 102 b enters the secondstator vane 203 b at the absolute velocity 303 d for the static fieldand exits therefrom at an absolute velocity 303 e as decelerated by theeffect of the second stator vane 203 b. At this time, the air flowobtains as much theoretical total pressure rise as determined by theequation 2.

FIG. 8 is a graph showing performance curve and resistance curve of theaxial flow fan.

Referring to FIG. 8, the air volume of the axial flow fan is generallydetermined by an operating point defined by intersection of acharacteristic curve 401 representing a relation between air volume andpressure loss in an operating environment of the axial flow fan and acharacteristic curve 402 representing a relation between air volume andpressure specific to the axial flow fan. Therefore, the fact that thestatic pressure rise is obtained due to the effect of the second statorvane indicates that the above characteristic curve of air volume versuspressure is converted to a characteristic curve 403 of air volume versuspressure. As a result, the operating point is shifted toward larger airvolume. Namely, the air volume is increased.

In this embodiment, if the first axial fan unit 1 shown in FIG. 1 fails,the first axial flow fan unit 1 makes an obstacle. At this time, thesecond axial flow fan unit 2 is operated at the maximum revolving speed.

As shown in FIG. 6, the negative preswirl is applied to the second rotorblade 102 b by the effect of the first stator vane 203 a, whereby theair flow can obtain a greater theoretical total pressure rise than in acase where the negative preswirl, expressed by the equation 1, is notapplied to the second rotor blade. Further, the effect of the secondstator vane 203 b provides a larger air volume than in a case where thesecond honeycomb 4 of FIG. 1 is omitted. That is, in the event of afailure of the first axial flow fan unit 1, the drop of air volume canbe reduced.

According to this embodiment as described above, the first stator vane203 a and the second stator vane 203 b have different configurations sothat the axial flow fan can achieve not only the reduced noise level andthe increased air volume but also the effect to suppress the failureinduced degradation of performance.

Now, description is made on a case where the first axial flow fan unit 1and the second axial flow fan unit 2, described with reference to FIG. 1illustrating the first embodiment of the invention, rotate in differentdirections.

A set of two axial flow fan units arranged in tandem and rotated in thedifferent directions is generally called a duplicate contra-rotatingfan. In this duplicate contra-rotating fan, an air flow through an axialflow fan unit on the upstream side in the air flow direction contains aswirling flow, which acts as the negative preswirl to the downstream fanunit. Hence, the pressure rise increased by the negative preswirl, asdescribed in the first embodiment, can always be prospected.

However, if inflow condition for the air into the upstream axial flowfan unit varies due to the change in the operating environment or thelike so that the negative preswirl to the downstream axial flow fan unitis increased too much, an air flow along a dorsal side of the bladebecomes unable to withstand such a large pressure rise and sustains flowseparation. This results in pressure loss.

Second Embodiment

According to a second embodiment, therefore, there are provided thefirst rotary rotor blade 102 a and a second rotary rotor blade 102 c. Inthe structure wherein the second rotor blade 102 c rotates in thedifferent direction, the first stationary stator vane 203 a isconfigured to direct a dorsal side thereof against the rotationdirection of the first rotor blade 102 a, while the second stationarystator vane 203 b is configured to direct the trailing edge thereof inparallel to the air flow direction.

Referring to FIG. 10, the operation of this embodiment is described asbelow.

FIG. 10 represents a cylindrical plane containing an axial flow fanaccording to the second embodiment of the invention.

Referring to FIG. 10, the air flow through the first stator vane 203 ahas the absolute velocity 303 c for the static field. The airflow entersthe second rotor blade 102 c at the relative velocity 302 d for therotational field and at the absolute velocity 303 d for the staticfield. At this time when the air flow enters the second rotor blade 102c, the first stator vane 203 a acts to prevent an excessive increase ofthe negative preswirl. Hence, the pressure loss caused by the air flowseparation is prevented while the theoretical total pressure riseexpected from the equation 1 may preferably be achieved. As illustratedby the first embodiment, the air flow through the second rotor blade 102c is increased in the static pressure by the effect of the secondstationary stator vane 203 b.

As described above, this embodiment affords an effect to suppress theloss encountered by the axial flow fan or more particularly theduplicate contra-rotating fan by virtue of the structure wherein thestator vane 203 a of the first honeycomb 2 and the stator vane 203 b ofthe second honeycomb 4 have different configurations from those of thestator vane 203 a of the first honeycomb 2 and the stator vane 203 b ofthe second honeycomb 4 shown in FIG. 1.

Third Embodiment

A third embodiment of the invention is described with reference to FIG.11.

FIG. 11 is a diagram showing a structure of an axial flow fan includingaxial flow fan units and honeycombs according to a third embodiment ofthe invention.

Referring to FIG. 11, the embodiment has the structure including thefirst axial flow fan unit 1, the first honeycomb 2, a second honeycomb 2a, the second axial flow fan unit 3 and a third honeycomb 4 which arearranged in the order starting from the upstream side in the air flowdirection indicated by the arrows.

FIG. 12 represents a cylindrical plane containing the inline axial flowfan according to the third embodiment of the invention.

Referring to FIG. 12, this embodiment has the structure wherein thefirst stationary stator vane 203 a is warped at a leading edge thereofagainst the rotation direction of the first rotary rotor blade 102 a,wherein the second stationary stator vane 203 b is warped at a trailingedge thereof against the rotation direction of the first rotor blade,and wherein the third stationary stator vane 203 c is configured todirect a trailing edge thereof in parallel to the air flow direction.

In other words, the first stator vane 203 a and the second stator vane203 b are two parts that form the first stator vane 203 a described inthe first embodiment shown in FIG. 6. If the “U” shaped stator vane 203a is to be formed in an integral mold, the molded product may have sucha configuration as not to be demolded. In this embodiment, therefore,the stator vane is formed of two separate parts, such as to facilitatethe molding process.

Next, the operation of this embodiment is described with reference toFIG. 12.

The air flow through the first rotor blade 102 a enters the first statorvane 203 a at an absolute velocity 301 b for the static field. The airflow passing through the first stator vane 203 a via the static fieldenters the second stator vane 203 b at the absolute velocity 303 c, theswirl component of which is reduced in the static field. The air flowthrough the second stator vane 203 b has the absolute velocity 303 d,which contains the negative preswirl against the second rotary rotorblade 102 b. When the negative preswirl is applied to the air flowentering the second rotor blade 102 b, the swirling flow of the air flowis reduced by the effect of the first stator vane 203 a. Thus isobtained an effect to suppress the production of flow separation fromthe air flow passing through the second stator vane 203 b. As a result,the loss caused by the air flow separation can be reduced or preferablyeliminated.

As described above, this embodiment has the structure wherein the statorvane 203 a of the first honeycomb 2 and the stator vane 203 b of thesecond honeycomb 2 a, shown in FIG. 11, have the differentconfigurations. Therefore, the embodiment can afford the effect tosuppress the loss caused by the flow separation from the air flow, theflow separation occurring when the negative preswirl is applied to theair flow into the second axial flow fan unit 3 by means of thehoneycomb.

Another advantageous effect of this embodiment is that the first statorvane 203 a and the second stator vane 203 b can be relatively easilyformed by molding, as described above.

Fourth Embodiment

A fourth embodiment of the invention is described with reference to FIG.13.

FIG. 13 is a diagram showing a structure of an axial flow fan includingaxial flow fan units and honeycombs according to a fourth embodiment ofthe invention.

Referring to FIG. 13, the embodiment has the structure including thefirst honeycomb 2, the first axial flow fan unit 1, the second honeycomb4, the second axial flow fan unit 3 and a third honeycomb 5 which arearranged in the order starting from an upstream side in the air flowdirection indicated by the arrows.

As shown in FIG. 14 representing a cylindrical plane containing thestructure shown in FIG. 13, the first stationary stator vane 203 a isconfigured to be warped at a trailing edge thereof in the rotationdirection of the first rotary rotor blade 102 a. The second stationarystator vane 203 b is configured to be warped in a “U” shape against therotation direction of the second rotary rotor blade 102 b. The thirdstationary stator vane 203 c is configured to direct a trailing edgethereof in parallel to the air flow direction.

Next, the operation of this embodiment is described with reference toFIG. 14.

The air flow passes through the first stator vane 203 a to obtain theabsolute velocity 303 b for the static field before entering the firstrotor blade 102 a. Since an operating environment assumed in a designphase differs from an actual operating environment, the loss may becaused by the air flow separation which may occur depending upon the airinflow condition varied due to the change in the operating environment.A function of the first stator vane 203 a is to reduce or preferably toeliminate this loss.

As described above, this embodiment has the structure wherein the statorvane 203 a of the first honeycomb 2, the stator vane 203 b of the secondhoneycomb 2 a and the stator vane 203 c of the third honeycomb 5, shownin FIG. 13, have the different configurations. Therefore, the embodimentcan afford the effect to prevent the loss resulting from the air inflowcondition varied due to the change in the operating environment.

Fifth Embodiment

A fifth embodiment of the invention is described with reference to FIG.15.

FIG. 15 is a schematic diagram showing a structure of a blade serveraccording to the fifth embodiment of the invention.

Referring to FIG. 15, a blade server 500 includes a casing 501, serverblades 502 arranged in the casing, and cooling fan modules 503 forcooling the server blades.

According to the invention, the structure of the first embodiment, forexample, may be adopted to form the cooling fan module 503 so that ablade server can attain high air volume and low noise by virtue of theeffects of the first embodiment. It is also possible to provide thecooling fan module excellent in redundancy in the event of a failure.

The applications of the cooling fan module according to the embodimentinclude, but are not limited to the blade server, all kinds of serverapparatuses such as rack mount servers and PC servers.

According to the invention as described above, a notable noise reductioncan be achieved because the effect of the first honeycomb permits thesecond axial flow fan unit to be reduced in the revolving speed. Inaddition, the cooling fan module is increased in the air volume becausethe static pressure is increased due to the effects of the firsthoneycomb and the second honeycomb.

In addition, if the first axial flow fan unit should fail, the drop ofcooling capacity can be reduced because the second honeycomb isprovided.

On the other hand, the first honeycomb acts to prevent the air flowseparation occurring in the second axial flow fan unit, therebysuppressing the generation of loss. Furthermore, the first honeycombalso acts to reduce the loss resulting from the varied inflow conditionof the air into the first axial flow fan unit.

In addition, the provision of the axial flow fan featuring the low noiseand high air volume makes it possible to fabricate a cooling fan modulefor server that is excellent in redundancy in the event of a failure.

1. An axial flow fan comprising: a first axial flow fan unit disposed onan upstream side with respect to an air flow; a first honeycomb disposeddownstream of the first axial flow fan unit; a second axial flow fanunit disposed downstream of the second honeycomb; and a second honeycombdisposed downstream of the second axial flow fan unit, wherein a statorvane constituting the first honeycomb is configured to be warped againsta rotation direction of the first axial flow fan unit, while a statorvane constituting the second honeycomb is configured to direct atrailing edge thereof in parallel to a direction of the air flow.
 2. Theaxial flow fan according to claim 1, wherein the stator vaneconstituting the first honeycomb is warped in a “U” shape.
 3. The axialflow fan according to claim 1, wherein the stator vane constituting thefirst honeycomb is divided into two parts.
 4. An axial flow fancomprising: a first axial flow fan unit disposed on an upstream sidewith respect to an air flow; a first honeycomb disposed downstream ofthe first axial flow fan unit; a second axial flow fan unit disposeddownstream of the first honeycomb; and a second honeycomb disposeddownstream of the second axial flow fan unit, the second axial flow fanunit rotating in a different way from the first axial flow fan unit,wherein a stator vane constituting the first honeycomb is configured todirect a ventral side thereof against a rotation direction of the firstaxial flow fan unit, while a stator vane constituting the secondhoneycomb is configured to direct a trailing edge thereof in parallel toa direction of the air flow.
 5. The axial flow fan according to claim 1,comprising an inline axial flow fan that uses the first and second axialflow fan units and the first and second honeycombs as a cooling devicefor server apparatuses.
 6. The axial flow fan according to claim 2,comprising an inline axial flow fan that uses the first and second axialflow fan units and the first and second honeycombs as a cooling devicefor server apparatuses.
 7. The axial flow fan according to claim 3,comprising an inline axial flow fan that uses the first and second axialflow fan units and the first and second honeycombs as a cooling devicefor server apparatuses.
 8. The axial flow fan according to claim 4,comprising an inline axial flow fan that uses the first and second axialflow fan units and the first and second honeycombs as a cooling devicefor server apparatuses.